MIMO (multiple-input multiple-output) refers to a method for improving data transmission/reception efficiency by adopting multiple transmit (Tx) antennas and multiple receive (Rx) antennas instead of a single Tx antenna and a single Rx antenna. When a single antenna is used, a receiver receives data through a single antenna path. However, when multiple antennas are used, the receiver receives data through multiple paths. Accordingly, a data transfer rate, throughput or signal-to-interference-and-noise-ratio (SINR) can be increased and coverage can be improved. A case in which both a transmitter and a receiver use a single antenna is referred to as SISO (Single-Input Single-Output), a case in which only the transmitter uses a plurality of antennas is referred to as MISO (Multiple-Input Single-Output), and a case in which only the receiver uses a plurality of antennas is referred to as SIMO (Single-Input Multiple-Output). In general, MIMO, SIMO, MISO and SISO schemes are commonly called MIMO technology.
To increase a multiplexing gain of MIMO operation, channel state information (CSI) can be fed back from an MIMO receiver and used for a MIMO transmitter. The receiver can determine the CSI by performing channel measurement using a predetermined reference signal (RS).